1. Field of the Invention
The present invention relates to a magnetic recording apparatus and relates to, in particular, a magnetic recording apparatus using an exchange coupled composite (ECC) medium.
2. Description of the Related Art
In hard disk devices, an improvement has been demanded in the performance of magnetic recording media to improve a magnetic recording density. When increasing the magnetic recording density, it needs to decrease the sizes of magnetic grains configuring a medium in order to maintain a signal quality (signal to noise (S/N) ratio) necessary for reproduction. However, when the sizes of the magnetic grains are decreased, the volume of the magnetic grains decreases and thereby magnetization loss due to thermal fluctuation becomes more likely to occur. In order to prevent this and to maintain a stable recording state, it needs to enhance magneto crystalline anisotropy energy (hereinafter, may be simply referred to as Hk) of the magnetic grains. However, when a material with large magneto crystalline anisotropy energy is used, an internal magnetic field (coercive force Hc and saturation magnetic field Hs) of the magnetic recording medium becomes large so that a strong recording magnetic field is needed to record to the magnetic recording medium. On the other hand, because the strength of a magnetic field that a magnetic head generates is limited by a material and a shape of a magnetic head, the recording may become difficult.
As a method to solve such a drawback, an ECC magnetic recording medium is discussed. The ECC magnetic recording medium includes a magnetic layer with small magneto crystalline anisotropy energy (hereinafter, referred to as a low Hk magnetic layer), a magnetic layer with large magneto crystalline anisotropy energy (hereinafter, referred to as a high Hk magnetic layer), and a nonmagnetic metal layer sandwiched between these magnetic layers. The low Hk magnetic layer and the high Hk magnetic layer are exchange-coupled through the nonmagnetic metal layer, and configure a recording layer together. By disposing the low Hk magnetic layer in a magnetic head side that is an upper layer side of the recording medium, a magnetization of the low Hk magnetic layer can be oriented in a magnetization hard axis direction (direction orthogonal to a magnetization easy axis) by a small magnetic field. A magnetization direction of the high Hk magnetic layer is oriented in the magnetization hard axis direction due to an exchange coupling force with the low Hk magnetic layer. Thereby, a magnetization direction of the recording layer can be oriented in a preferred direction by a small recording magnetic field, and thermal stability of the recording layer can be maintained. The specification of U.S. Patent Application Publication No. 2011/0122525 should be referred.
In the ECC magnetic recording medium, it is necessary to maintain a balance between the strength of a necessary recording magnetic field and thermal stability. In other words, when Hk of the low Hk magnetic layer is large, the necessary recording magnetic field becomes large and thereby an effect on a configuration of the magnetic head increases. When Hk of the low Hk magnetic layer is small, it becomes difficult to rotate the magnetization direction of the high Hk magnetic layer and thereby a necessity to lower Hk of the high Hk magnetic layer occurs. As a result, the thermal stability of the entire recording layer declines and this restricts the higher recording density. As described above, in the ECC magnetic recording medium, flexibility of a film configuration is low and there are also limitations to a coping ability of the magnetic head.
It is objective of the present invention to provide a magnetic recording apparatus that is provided with an ECC magnetic recording medium as a magnetic recording medium, that can realize a large recording density with a small recording magnetic field, and that can increase flexibility of a film configuration of the ECC magnetic recording medium.